The first-generation Galaxy Tab S hails from a time when Samsung was still using swipe-based fingerprint scanners - yeah, it's that old. Just look at that stock wallpaper; Galaxy S5 flashbacks, anyone? Around three weeks ago, Verizon updated its 10.5" Tab S to Marshmallow, and now AT&T is doing the same. The Death Star is also throwing in support for NumberSync.
Yes, we know, Android 6.0 Marshmallow is over a year old now, but at least this build (MMB29K.T807AUCU1CPI8) carries the relatively recent September 1st security patch. This update also brings NumberSync functionality, which essentially allows AT&T customers to use their tablets to make and receive voice and video calls, even when they're not near their smartphones, with their existing phone numbers.
The update weighs in at a hefty 1063MB, so make sure that you have a solid WiFi connection before downloading it. There's still no word on Marshmallow for the AT&T variant of the 8.4" model.